1. Field of the Invention
Embodiments of the present invention relate in general to combustors and, more particularly, to premixing devices with high expansion fuel injection slot jets for enhanced mixing of fuel and oxidizer in low-emission combustion processes.
2. Description of the Related Art
Historically, the extraction of energy from fuels has been carried out in combustors with diffusion-controlled (also referred to as non-premixed) combustion where the reactants are initially separated and reaction occurs only at the interface between the fuel and oxidizer, where mixing and reaction both take place. Examples of such devices include, but are not limited to, aircraft gas turbine engines and aero-derivative gas turbines for applications in power generation, marine propulsion, gas compression, cogeneration, and offshore platform power to name a few. In designing such combustors, engineers are not only challenged with persistent demands to maintain or reduce the overall size of the combustors, to increase the maximum operating temperature, and to increase specific energy release rates, but also with an ever increasing need to reduce the formation of regulated pollutants and their emission into the environment. Examples of the main pollutants of interest include oxides of nitrogen (NOx), carbon monoxide (CO), unburned and partially burned hydrocarbons, and greenhouse gases, such as carbon dioxide (CO2). Because of the difficulty in controlling local composition variations in the flow due to the reliance on fluid mechanical mixing while combustion is taking place, peak temperatures associated with localized stoichiometric burning, residence time in regions with elevated temperatures, and oxygen availability, diffusion-controlled combustors offer a limited capability to meet current and future emission requirements while maintaining the desired levels of increased performance.
Recently, lean premixed combustors have been used to further reduce the levels of emission of undesirable pollutants. In these combustors, proper amounts of fuel and oxidizer are well mixed prior to the occurrence of any significant chemical reaction, thus facilitating the control of the above-listed difficulties of diffusion-controlled combustors. However, because a combustible mixture of fuel and oxidizer is formed before the desired location of flame stabilization, premixed combustor designers are continuously challenged with the control of any flow separation and/or flame holding in the regions where mixing takes place so as to minimize and/or eliminate undesirable combustion instabilities. Current design challenges also include the control of the overall length of the region where mixing of fuel and oxidizer takes place and the minimization of pressure drop associated with the premixing process. These challenges are further complicated with the need for combustors capable of operating properly with a wide range of fuels, including, but not limited to, natural gas, hydrogen, and synthesis fuel gases (also known as syngas), which are gases rich in carbon monoxide and hydrogen obtained from gasification processes of coal or other materials.
Conventional premixed burners incorporate fuel jets positioned between vanes of a swirler or on the surface of the vane airfoils. However, vortical structures formed at the fuel jet exits tend to pull oxidizer from the free stream under the fuel jet, resulting in the partial or total “blow-off” of the flow near the surface and creating a separation region in the main flow that could lead to premature ignition. In addition, this cross-flow injection of fuel generates localized regions of high and low concentrations of fuel/air mixtures within the combustor, thereby resulting in substantially higher emissions. Further, such cross-flow injection results in fluctuations and modulations in the combustion processes due to the fluctuations in the fuel pressure and the pressure oscillations in the combustor that may result in destructive dynamics within the combustion process. Recently, premixing devices using Coanda surfaces have been proposed as a way to minimize the negative effects of premixed combustors that depend primarily on cross-flow fuel injection to achieve a desired level of premixing and overall performance. In these devices, fuel injected along a Coanda surface adheres to the surface as the mainstream airflow is accelerated, preventing liftoff and separation of the fuel jets as well as undesirable pressure fluctuations that may cause combustion instability. In premixing devices with Coanda surfaces, the efficient mixing of the fuel with the oxidizer may be somewhat delayed since the fuel jet is maintained next to a diverging wall, thus potentially resulting in devices that are long in order to assure proper mixing of fuel and oxidizer. If the length of the premixing device is constrained by an overall engine length requirement, for example, the fuel concentration profile delivered to the flame zone may contain unwanted spatial variations, thus minimizing the full effect of premixing on the pollutant formation process as well as possibly affecting the overall flame stability in the combustion zone.
The undesirable effects of over-expansion in diverging flow passages is common knowledge in fluid mechanics; however, the use of a converging-diverging fuel injection slot jet with controlled localized flow separation with the intent of generating turbulence and fluid mixing at an injection site is unknown to this inventor. Therefore, a need exist for a premixing device for use in lean-premixed combustors with enhanced capabilities of mixing fuel and oxidizer while maintaining control of flow separation and flame holding in the mixing region of the combustor. The increased mixing performance will permit the development of premixing devices having a reduced length without substantially affecting the overall pressure drop of the system, premixed combustors incorporating such premixers being particularly suitable for use with fuels having a wide range of composition, heating values and specific volumes.